Xirix, from what I gather, you want to have a second type of Marine buildable from the Barracks who has shields.

Starcraft modding is often done through replacement. What you replace, though, is often very flexible. Creating a second Marine with shields would be simple, and you needn't use Jim. Using a DAT editor like DatEdit or PyMS, use the Copy command on the Marine, pick an unused Units.dat entry, and paste the Marine data. This will set the unused entry to use the Marine's Iscript animations, GRP graphics, sounds, portrait, everything. At this point, feel free to give this new marine his shields.

The only part left is now is allowing the Barracks to build the new Shield Marine and creating the button for it. This will be done with FireGraft. Are you familiar with FG?